1. Field of the Invention
The present invention relates to a flexible board obtained by forming a polyimide insulating layer of a three-layer structure on metal foil.
2. Related Art of the Invention
Products as obtained by pasting a single-layer polyimide film (insulating layer) to copper foil by thermocompression bonding with the aid of adhesives (for example, epoxy-based adhesives or urethane-based adhesives) are widely used as typical single-sided flexible boards.
Since copper foil and polyimide films are not the same in terms of their coefficients of linear thermal expansion, cooling such single-sided flexible boards following thermocompression bonding sometimes causes curling, twisting, warping, or the like.
In view of this, it has been proposed (Japanese Patent No. 2746555) to use flexible printed boards (copper-clad products) in which curling is suppressed by using a laminated polyimide-based insulating layer having a three-layer structure. These flexible printed boards have laminated polyimide-based resin layers whose three-layer structure on a copper foil used as a conductor comprises a first polyimide-based resin layer that has a coefficient of linear thermal expansion of 20xc3x9710xe2x88x926 /K or greater and that is formed by means of coating on the copper foil side, a second polyimide-based resin layer that has a coefficient linear thermal expansion of less than 20xc3x9710xe2x88x926 /K and that is formed by means of coating in the same manner on the first layer, and a third polyimide-based resin layer that has a coefficient of linear thermal expansion of 20xc3x9710xe2x88x926 /K or greater and that is further formed by means of coating on the second layer. Here, the specific relation among the coefficients of linear thermal expansion of the first polyimide-based resin layer (coefficient of linear thermal expansion: k1), second polyimide-based resin layer (coefficient of linear thermal expansion: k2), and third polyimide-based resin layer (coefficient of linear thermal expansion: k3) is expressed as k3xe2x89xa7k1 greater than k2, as shown in Table 1 below (see Working Examples 8 to 12 and Table 2 in Japanese Patent No.2746555). In addition, the coefficient of linear thermal expansion (k0) of a laminated film (substantially corresponds to a laminated polyimide-based resin film having a three-layer structure) following conductor etching is 11xc3x9710xe2x88x926 /K.
The flexible printed board described in Japanese Patent 2746555, however, has a large difference in the coefficient of linear thermal expansion (8 to 11xc3x9710xe2x88x926 /K) between the electrolytic copper foil (coefficient of linear thermal expansion following polyimidation heat treatment: about 19xc3x9710xe2x88x926 /K ) and the laminated polyimide-based resin layer coefficient of linear thermal expansion: about 9 to 11xc3x9710xe2x88x926 /K), so the board undergoes substantial curling, twisting, or warping following copper foil etching when the flexible printed board is cooled to room temperature and allowed to shrink following imidation heat treatment. In addition, the coefficient of linear thermal expansion of copper foil is greater than the coefficient of linear thermal expansion of the laminated polyimide-based resin layer, so the board contracts and curls such that a convexity is formed on the laminated polyimide-based resin layer side. Furthermore, the coefficient of linear thermal expansion of the third polyimide-based resin layer is equal to or greater than the coefficient of linear thermal expansion of the first polyimide-based resin layer, so the curling of the entire laminated polyimide-based resin layer fails to produce a convexity on the copper foil side despite the fact that the presence of a convex curl on the copper foil side is preferred, taking into account the lamination of a cover layer on a circuit pattern following the formation and treatment of this circuit pattern.
Thus, a resulting drawback is that when curling occurs in such a way that a convexity forms on the laminated polyimide-based resin layer side of the flexible printed board, it is difficult to hold the board by vacuum chucking (with the copper foil side facing up) in order to pattern the copper foil, and when the board is transported along a line, the ends of the board curl up and become caught or otherwise damaged in a transporting device during transport.
An object of the present invention is to overcome the above-described drawbacks of the prior art and to provide a flexible board obtained by forming a laminated polyimide-based resin layer having a three-layer structure and serving as an insulating layer on metal foil, wherein this flexible board is configured such that the curling is absent or is negligibly small following metal foil patterning or imidation heat treatment, and that a convexity is formed on the metal foil side when slight curling does occur.
The present inventors perfected the present invention upon discovering that the stated object can be attained by arranging each of the polyimide-based resin layers constituting a laminated polyimide-based resin layer of a three-layer structure in the following descending order of coefficients of linear thermal expansion: the polyimide-based resin layer on the metal foil side, the polyimide-based resin layer on the side opposite the metal foil, and the polyimide-based resin layer sandwiched therebetween.
Specifically, the present invention provides a flexible board, comprising a metal foil and provided thereon a laminated polyimide-based resin layer of a three-layer structure comprising a first polyimide-based resin layer, a second polyimide-based resin layer, and a third polyimide-based resin layer, wherein said flexible board satisfies the equation
k1 greater than k3 greater than k2
where k1 is the coefficient of linear thermal expansion of the first polyimide-based resin layer on the metal foil side, k2 is the coefficient of linear thermal expansion of the second polyimide-based resin layer, and k3 is the coefficient of linear thermal expansion of the third polyimide-based resin layer.
These and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.